It has been proposed, for example in UK Pat. No. 1522601 and in U.S. Pat. No. 4267801, to remove material from a fluidisable bed, to remove ash or other particles from that material and to return the remainder to the bed.
However, both of those proposals were directed to the removal of unwanted relatively larger particles from a relatively deep bed and in which the whole of the bed is given motion. Furthermore, in those proposals there was no requirement, and no provision, for the recovery of bed material carried away from the bed by gas stream leaving the bed.
The present invention is concerned with fluidised bed combustion apparatus in which in a containment for a bed of material means for introducing fluidising gaseous fluid into the material are arranged so as to fluidise material only in an upper zone of the containment and so as to leave in a lower zone of the containment a layer of quiescent material. The quiescent layer protects the boundary of the containment beneath the layer.
In such apparatus the depth of bed material in the containment is relatively small. In order that the accumulation of incombustible residues from fuel shall not rise to excessive levels it is necessary to remove a mixture of bed material and such accumulating residues from the containment. A further proportion of material containing a mixture of at least bed material and relatively smaller, buoyant particles of incombustible matter from fuel leaves the containment in the gas stream flowing away from the containment.
Accordingly, the following requirements arise:
(1) the materials in the gas stream will ultimately fall out from the gas stream and accumulate elsewhere in the apparatus. That accumulation must be removed from the location at which it occurs;
(2) bed material is lost at a significant rate from the containment and as much as practicable must be recovered from the mixtures aforesaid and returned to the containment, while as high a proportion as practicable of incombustible matter from fuel and particularly relatively larger particles, is removed from the apparatus; and
(3) the removal of incombustible residues accumulated in the containment must be such as to leave enough of the quiescent layer to protect at least the vulnerable part of the underlying boundary of the containment.
The teaching of the prior proposals referred to above is not applicable to such apparatus and cannot meet the requirements which have just been explained, whereas the apparatus according to the invention does meet those requirements.